A conventional method for soldering the terminals of an electronic component to a circuit carrying substrate as practiced in the prior art is shown in FIG. 1. In a typical situation, solder paste 3 is printed or otherwise deposited on the desired pads 2 of printed circuit substrate 1. Those skilled in the art will appreciate that solder paste is not applied to all the solderable surfaces of the printed circuit substrate 1 but only to selected solderable surfaces desired to be connected to electrical components. An electrical component 4 is then placed on the printed circuit substrate 1 so that the electrical terminations 5 are in direct contact with the solder paste 3. Although FIG. 1 shows an electronic component 4 having leaded terminations 5, other configurations of electronic components, for example, leadless components are well known. The assembly, consisting of the substrate, electrical component, and the solder paste is then heated or reflowed to melt the solder paste 3 and cause it to wet to the pads 2 and the terminations 5, thereby creating a fillet or electrical interconnection between the component and substrate.
In high density, high volume applications, it is desirable to eliminate solder paste because of the difficulty and cost associated with printing small areas of solder paste. Cladding solder to the desired pads or printed circuit board is a substitute for solder paste. An area of solder is built up or deposited on the pads and the component terminations are placed directly on the solder cladding. One of the problems associated with this type of assembly is that the surface of the clad solder 3 is not perfectly flat and is typically rounded due to surface tension of the solder. Placing the component on this round solder mound 3 causes the component to slide off the pads and become misaligned, thereby creating defects during the soldering operation.
It would be highly desirable if one were able to eliminate the need for solder paste while still retaining the advantages of placing the component on a flat, uniform surface. Such a method would allow one to produce high density electronic circuits with significantly fewer manufacturing defects.